The present invention relates to a method for piloting a braking system of a vehicle, in particular, this method is adapted to be implemented with a braking system comprising at least a pair of coupled brake actuators, that includes a pneumatic actuator and an electric actuator. The invention also concerns a braking system associated with this method and a vehicle, such as a truck, equipped with such a braking system.
The invention can be applied in heavy-duty vehicles, such as trucks, buses and construction equipment. Although the invention will be described with respect to a truck, the invention is not restricted to this particular vehicle, but may also be used in other vehicles, such as tractors and vans.
In the field of brake actuation, it is known to use actuators working with two different energy sources. An actuator of this kind may be known as a “bi-energy actuator”.
For instance, WO-A-03/014588 discloses a disk brake caliper working both with hydraulic and electric energy. Indeed, this disk brake caliper includes a hydraulic service brake actuator and an electric parking brake actuator. Two disk brake pads are located on either side of a brake disc to be braked. The hydraulic actuator includes a piston disposed within a cylinder. A thrust chamber is delimited between the rear of the piston and a moving casing of the caliper. When fluid is injected in the trust chamber, the piston moves towards the brake disc under the hydraulic pressure and pushes a first disk brake pad against the brake disc. Simultaneously, the casing of the caliper moves in the opposite direction under the hydraulic pressure and a second disk brake pad, which is secured to the moving casing of the caliper, comes into contact with the brake disc. Consequently, the brake disc is squeezed between the two disk brake pads and is slowed down by friction. The electric actuator produces the same disk brake pad shifting movement than the pneumatic actuator. It may be activated alone or in cooperation with the hydraulic actuator. The electric actuator includes a rod for pushing the first disk brake pad against the brake disk. The rod is in threaded engagement with a mated female thread formed on the internal surface of a bore delimited in the casing. The rod is translated by means of a worm gear that is in meshed engagement with another worm gear driven by an electric motor. The rotation of the rod inside the bore involves the rod to move towards the brake disk and the casing to move in the opposite direction. Consequently, the brake disc is squeezed between the two disk brake pads, thereby inducing friction and the braking action.
WO-A-2008/030204 also discloses a “bi-energy” actuator. In particular, the second design disclosed in this document corresponds to an electro-pneumatic brake actuator that comprises a pneumatic brake actuator powered by compressed air and an electric brake actuator that is powered by electricity. The pneumatic brake actuator comprises a casing that delimits a service chamber. The service chamber is in contact with a diaphragm made of rubber. When filling the service chamber with compressed air, the diaphragm overturns and pushes a flange connected to a drive rod, which implies braking. The electric brake actuator is realized by an electric motor whose outlet shaft is connected, by means of a set of gears, to the drive rod of the brake. In parking conditions, the electric brake actuator and the pneumatic brake actuator are operated in parallel while in service conditions, the pneumatic brake actuator and the electric brake actuator are operated consecutively, with the pneumatic brake actuator being operated first. Both brake actuators are operated in a complementary manner, so as to exert an adequate braking force.
The two above mentioned documents disclose a bi-energy actuator but do not disclose a method for piloting it in case of an accidental depressurization of the compressed air source that supplies the pneumatic actuator, which corresponds to a degraded mode.
Each pneumatic actuator of a vehicle is supplied with a main compressed air source that is boarded on the vehicle. In case of a truck, the brakes, the gear box and the wheels suspension can also be powered with compressed air. The compressed air source is typically an air tank that is pressurized at an adequate pressure by a compressor. In operation, the air pressure in the air tank decreases when operating one or more pneumatic actuators. When the pressure in the air tanks falls at or under a cut-in pressure, the air tank pressure is build up, or “repressurized” up to a cut-off pressure that is to say to a upper pressure limit in the air circuit, preferably in the air tank, at which or above which the compressor is stopped or is put on a stand-by mode. The cut-in pressure corresponds to a lower pressure limit in the air circuit, preferably in the air tank, at which or under which the compressor has to be re-started in order to build up the pressure in the air tank.
However, if the compressor is malfunctioning or if a leakage occurs in the distribution air system, the pressure of the air tank may fall under the cut-in pressure, without the compressor being able to build up the air tank pressure. In this case, the pneumatic actuators, such as pneumatic brake actuators, may run out of energy to operate. This is dangerous because the driver is then unable to slow down the vehicle.
It is desirable to provide a method for piloting a bi-energy braking system, which takes into account a possible malfunctioning of the compressor or an air leakage in the air distribution system, by operating a degraded mode. In this degraded mode, the method according to an aspect of the invention allows saving pneumatic energy by activating the brake electric actuator, which operates alone or in a complementary manner with the brake pneumatic actuator. As far as possible, an aspect of the method aims at keeping a certain amount of energy to power the essential actuators of the vehicle, such as, when the vehicle comprises a truck and a trailer, for powering the brakes of the truck and the brakes of the trailer.
To this end, the invention concerns, according to an aspect thereof, a method for piloting a braking system of a vehicle. This braking system comprises at least a pair of coupled brake actuators that includes a pneumatic brake actuator supplied with compressed air coming from an air tank that is pressurized by a compressor, and an electric brake actuator. According to an aspect of the invention, this method comprises the steps of:
a) measuring the air pressure of the air tank.
b) if the air pressure measured at step a) is inferior to a first threshold value, assessing whether the compressor is able to build up the air pressure in the air tank,
c) if the result of the assessment performed at step b) is that the compressor is unable to build up the air pressure of the air tank, operating the braking system in a degraded mode wherein at least the electric brake actuator is used in case of new braking actions, and
d) if the air pressure measured at step a) is inferior to a second threshold value, that is lower than the first threshold value, and if, according to the result of step b), the compressor is unable to build up the air pressure of the air tank, using only the electric brake actuator in case of new braking actions.
By the provision of a method which comprises a degraded mode, when the compressor is malfunctioning or when an air leakage occurs in the air distribution system, the electric brake actuator of the actuating system takes over the pneumatic actuator so as to save pneumatic energy. Depending on the air pressure level measured in the air tank, the electric actuator may be used alone or in cooperation with the pneumatic actuator.
According to further aspects of the invention which are advantageous but not compulsory, such a method may incorporate one or several of the following features:                If the air pressure measured at step a) is between the first and the second threshold values, then the pneumatic brake actuator and the electric brake actuator are used together in case of new braking actions.        Steps c) and d) may include a sub-step a) that consists in or comprises measuring the state of charge of a vehicle battery that is normally used to power the electric brake actuator according to step c) or d).        Steps c) and d) include a sub-step β) posterior to step a), that consists in or comprises, if the state of charge of the vehicle battery is lower than a predetermined value, activating as backup battery to power the electric brake actuator according to step c) or d).        Preferably, following the sub-step β) a predetermined number of new braking actions is performed before a stopping rule of the vehicle is activated or following the sub-step β) a given distance is travelled by the vehicle before a stopping rule of the vehicle is activated.        The method may further comprise a step e) that consists in or comprises, if the air pressure measured at step a) is inferior to a third threshold value, that is lower than the second threshold value, and if, according to the result of step b), the compressor is unable to build up the air pressure of the air tank, using only the electric brake actuator in case of new braking actions and activating a stopping rule of the vehicle after the performance of a predetermined number of new braking actions or after the vehicle has traveled a given distance since the braking system has been operated in the degraded mode or since the air pressure has been detected as being inferior to the third threshold value.        A predetermined number of new braking actions can be performed at steps c) and/or d) before a stopping rule of the vehicle is activated.        The predetermined number of new braking actions allowable at step c) is preferably superior to that at step d).        In a variant, a stopping rule of the vehicle can be activated at steps c) and/or d) after the vehicle has traveled a given distance since the braking system has been operated in the degraded mode.        The stopping rule of the vehicle consists in or comprises immobilizing automatically the vehicle when safe standstill conditions of the vehicle are detected.        The safe standstill conditions of the vehicle are preferably detected when the speed of the latter falls below a certain value and/or when park brake activation is detected.        The first threshold value is lower than or equal to the cut-in pressure of the air tank, that is the pressure at which the air tank pressure is normally build up by the compressor.        Warning signals can be sent to the driver of the vehicle at steps c) and d).        
This invention also concerns a braking system of a vehicle, this braking system comprising at least a pair of coupled brake actuators that includes a pneumatic brake actuator supplied with a air tank pressured by a compressor, and an electric brake actuator. According to an aspect of the invention, this braking system includes:                means for automatically measuring the pressure of the air tank,        means to compare the measured pressure to a first threshold value and to a second threshold value and to assess whether the compressor is able to build up the air pressure of the air tank, these means being adapted to:        
i. if the measured pressure is inferior to the first threshold value and if the compressor is unable to build up the air pressure of the air tank, operate the braking system in a degraded mode wherein at least the electric brake actuator is used in case of new braking actions, and
ii. if the measured pressure is inferior to the second threshold value, that is lower than the first threshold value, and if the compressor is unable to build up the air pressure of the air tank, use only the electric brake actuator in case of new braking actions.
Finally, the invention relates according to an aspect thereof, to a vehicle equipped with a braking system as previously defined.